Exploration of marine-derived actinomycetes as a source of antitumor compounds has led to the isolation of a new member of the tartrolon series, tartrolon D (4), from fermentation broths of Streptomyces sp. MDG-04-17-069, which was isolated from a 30 m deep marine sediment collected off the east coast of Madagascar. Additionally, the known compound ikarugamycin (5) was also found in the culture broths of the same microorganism. The structure of this new tartrolon was established by a combination of spectroscopic techniques (1D and 2D NMR, HRMS, and UV) as well as by comparison with published data for similar compounds. Cytotoxicity assays were performed for tartrolon D against a panel of three human tumor cell lines, including lung (A549), colon (HT29), and breast (MDA-MB-231). The compound displayed GI50 values in the submicromolar range against all the lines tested, making it a promising candidate to pursue further studies to assess its potential as an anticancer agent.